In a point-to-multipoint communication system having two-way messaging capability, e.g., an acknowledge-back paging system, one way of accommodating confirmations is through forward and reverse channels that operate on a time-division multiplexed (TDM) basis on a shared common frequency. That is, the forward channel periodically transmits information for a first predetermined time followed by transmissions on the reverse channel for a second predetermined time. Alternatively, such systems can utilize two different frequencies for the forward and reverse channels. Typically, such systems also uniquely assign (e.g., via the forward channel transmissions) individual time slots for the multi-point communicators to use for sending messages on the reverse channel, so that multiple confirmation messages from different communicators do not collide. The multipoint communicators typically synchronize their time slot transmissions with the periodic transmissions received from the forward channel, so that a multi-point communicator assigned a time slot will transmit at substantially the same time (with respect to the forward channel timing) as any other multi-point communicator assigned the same time slot. Thus, a reverse channel receiver at the point station also can be synchronized with the periodic transmissions of the forward channel in order to be synchronized with a nominal predetermined timing for receipt of the time slot transmissions of the multipoint communicators.
A difficulty occurs, however, due to variable transmission delays between the point station and the multipoint communicators. Multipoint communicators that are very near the point station will experience very brief transmission delays, while those far from the point station will experience relatively long transmission delays. For example, a communicator thirty miles from the point station will experience a round-trip transmission delay of about 300 microseconds, while a communicator located immediately next to the point station will experience substantially zero transmission delay. For a system operating at 4800 symbols per second, for example, 300 microseconds of transmission delay is approximately 1.5 symbol periods. Because the amount of transmission delay experienced by any given portable multipoint communicator is unpredictable (because the multipoint communicator can move around), the reverse channel receiver at the point station is only approximately synchronized with the reverse channel messages.
To enable exact synchronization of the reverse channel messages for decoding, a synchronization word is transmitted at the beginning of each message. Preferably, for transmission efficiency the synchronization word is a short word comprising only a few, e.g., twelve, symbol periods. Also, because the reverse channel signals generated by the multipoint communicators typically use low-power transmitters, the point station reverse channel receiver must have high sensitivity. Unfortunately, a short synchronization word can easily be falsed by noise in a high sensitivity receiver, i.e., a noise burst can generate a symbol pattern that mimics a short synchronization word. Having been "fooled" by noise into detecting a false sync word, a less-than-robust receiver could continue to "decode" additional noise that is received after the false sync word, thereby interpreting the noise as valid packet data.
Furthermore, in a point-to-multipoint system with two-way messaging, many of the time slots of the reverse channel will often be empty during any given transmission cycle. The empty time slots will thus contain only noise. The point reverse channel receiver has no way of predicting which of the time slots contain valid data and which contain noise. This is because the point receiver does not know which of the multipoint communicators will respond to a forward channel transmission. Also, in addition to transmitting scheduled responses to messages received over the forward channel, the multipoint communicators can transmit unscheduled messages in at least some of the time slots.
Thus, what is needed is a method and apparatus that can minimize false detection of packet data in a receiver due to noise in data packet time slots, wherein the time slots have a nominal predetermined timing for receipt, which timing varies from the actual timing of receipt due to variable transmission delays, and wherein some of the time slots contain a transmitted signal comprising a data packet, while others of the time slots contain only noise.